A mobile unit communication system can keep communication by properly switching the base stations communicating with a mobile terminal when the mobile terminal changes its location. Such switching control over base stations is called handover.
The mobile terminal monitors the receive signal quality from a base station currently communicating and the receive signal quality from base stations surrounding the base station communicating. The mobile terminal monitors different receive signal qualities from the base station communicating and surrounding base stations with the movement of the mobile terminal. The mobile terminal implements handover under the condition that the state that the receive signal quality from a surrounding base station is higher than the receive signal quality from the base station communicating continues for a preset period of time. (Refer to Japanese Laid-open Patent Publication No. 2007-502558 and Japanese Laid-open Patent Publication No. 2008-92378, for example.)
However, mobile terminals in the past determine the start of handover in accordance with whether the condition has been satisfied or not, irrespective of the degree of changes in communication qualities from the base station communicating and surrounding base station. Thus, when the receive signal quality from the base station communicating rapidly deteriorates due to the high velocity of the movement of the mobile terminal or influences of buildings, for example, the timing of handover may delay. Also when the receive signal quality from a surrounding base station rapidly increases, the timing of handover may delay. When the timing of handover delays, a signal from the surrounding base station becomes interference waves to the base station communicating. Then, the signal quality from the base station communicating may deteriorate. This prevents the communication between the mobile terminal and the base station, possibly resulting in call disconnection in the mobile terminal.
In order to control so as to change the handover start condition in accordance with the pattern of changes in the surrounding environment, a handover start threshold value or offset value has been set for each pattern in mobile terminals in the past. This however has increased the capacity of the memory area for pre storing the handover start threshold values or offset values.